The present invention is concerned with the reduction of pollution in areas where thermoelectric power plants are present and also with an improvement in the environment of such plants. More particularly, the invention relates to the problem of removing sulfur dioxide from the smoke which develops as a result of the combustion of heavy mineral oils; sulfur dioxide forms as a result of the reaction between oxygen and the sulfur present in such oils.
The problems arising from the undesirable production of sulfur dioxide in thermoelectric power plants are of great concern; however, to the present time, these problems have not been solved in a satisfactory manner.
While methods involving the dispersion of smoke containing sulfur dioxide are known, such methods are carried out substantially by conveying the smoke to a sufficiently tall chimney where it is diluted and no longer dangerous. This solution may prevent the pollution of the area immediately surrounding the power plant, however it has inconveniences and in any event is not completely satisfactory. As a matter of fact, owing to the winds, the problem may simply be transferred from one area to another.
Methods based on the adsorption of sulfur dioxide on suitable supports and in the presence of catalytic agents of particular chemical nature also have been suggested. However, such methods have, in practice, both technical and economic inconveniences. Sulfur dioxide is highly diluted in the combustion gases and accordingly, a reliable removable process should include large contact surfaces and reaction beds of substantial height.
It is further known from U.S. Pat. Nos. 3,920,794 and 3,914,387 that sulfur dioxide may be absorbed in an alkaline solution, wherein the sulfur dioxide is converted into a sulfite, for example, sodium sulfite which is then oxidized to a sulfate, for example sodium sulfate, due to the action of oxygen and air in the presence of a catalyst which has been added to the absorption solution. The quantity of oxidation catalyst in the absorption liquid is in the range of some parts per million, a quantity which may be effective for catalytic activity, but which does not permit the catalyst to participate directly in the absorption reaction.
In these processes the absorption reaction, i.e. the reaction between sulfur dioxide and the alkaline reagent takes place countercurrently in washing towers, wherein the gas in fed from below, while the alkaline solution is fed from above. After contact with sulfur dioxide, the alkaline absorbent solution becomes neutral or acid; therefor it is evident that the adsorption reaction occurs mainly in the upper zone of the washing tower, because in the lower zone, the sulfur dioxide meets a neutral or slightly acid solution in which the absorption reaction does not occur. Thus, the total height of the washing tower is not actually used to absorb sulfur dioxide and accordingly, the process is inefficient.